PROJECT SUMMARY (Project 6; Brusseau, Field) Acid rock drainage and its potential impact on surface-water and groundwater contamination is generally considered to be a primary environmental concern for many hardrock mining sites in the USA. Concomitantly, cost-effective treatment of mining-impacted waters to levels protective of human health and the environment is a critical issue for mitigating the risk associated with hardrock mining sites. The overall goals of this project are to investigate the key physical and biogeochemical processes that control migration and attenuation of mine- drainage contaminants in groundwater, and to develop and test innovative methods for characterization and remediation of contaminated groundwater at hardrock mining sites. The specific aims of the project are: (1) Investigate the key processes that control migration and attenuation of mine-drainage contaminants at the source-groundwater interface; (2) Investigate the feasibility and long-term efficacy of in-situ biosequestration for the remediation of groundwater contaminated by acid-rock drainage; (3) Develop and apply integrated site characterization methods to evaluate groundwater contamination potential, assess risk, and determine the feasibility of remediation at mining sites. The project incorporates innovative hydrological and biogeochemical methods to develop approaches designed specifically for the unique properties and conditions inherent to mining sites in the Southwestern USA. The project targets the primary groundwater contaminants of concern at these sites, including sulfate, arsenic, selenium, perchlorate, and uranium. This project encompasses experiments conducted across several scales, from the pore-scale to the intermediate-scale to pilot-scale field tests. This project will advance the state of the science regarding the transport and attenuation of metalloids in the subsurface, and their remediation. For example, in-situ biosequestration is one of the very few options that exist for remediation of the large, deep groundwater contaminant plumes that form at hardrock mining sites. This method has great potential, but several critical questions exist as barriers to widespread acceptance and adoption. This project will answer these questions, with a specific focus on novel methods for implementing in- situ biosequestration under the conditions representative of sites in the Southwestern USA. It is anticipated that the application of project outcomes will produce significant cost savings for the clean-up of the nation's inventory of mine waste sites.